1. Field of the Invention
The present invention relates to a high-voltage power supply using a piezoelectric transformer and to an image forming apparatus using such a high-voltage power supply.
2. Description of the Related Art
In electrophotographic image forming apparatuses, it is necessary to provide a high-voltage power supply that generates voltages in excess of several hundred volts (high voltages), for example to apply a DC bias voltage to a transfer member. Conventional high-voltage power supplies have used wire-wound electromagnetic transformers, but in recent years, high-voltage power supplies have used piezoelectric transformers (piezoelectric ceramic transformers) as well.
Japanese Patent Laid-Open No. 11-206113 discloses a high-voltage power supply including a voltage-controlled oscillator (VCO) generating a drive frequency that is input into a piezeoelectric transformer. The piezoelectric transformer has the feature that the output voltage becomes maximal at a resonance frequency. Therefore, the high-voltage power supply disclosed in Japanese Patent Laid-Open No. 11-206113 controls the output voltage of the piezoelectric transformer by controlling the frequency that is output from the VCO.
Ordinarily, the VCO changes the drive frequency of the piezoelectric transformer from a high frequency to a low frequency for example, in order to increase the output voltage of the piezoelectric transformer. Moreover, the VCO controls the duration of the rising edge of the output voltage by controlling the rate of change of the drive frequency. For this reason, the higher the desired voltage is, the longer the duration of the rising edge becomes. Furthermore, since the drive frequency is changed, output ripple tends to occur in the constant voltage output of the piezoelectric transformer and unnecessary circuit oscillations tend to be generated. For example, there is the risk that in an image forming apparatus using such a high-voltage power supply, the quality of the formed images is deteriorated due to the ripple in the output.